Durability, strength, and reliability are goals that often oppose efficiency in the manufacturing of many sorts of structures. In order to be both commercially viable and safe, vehicle components in particular should be both light weight and structurally sound. In meeting both challenges, modern vehicle components such as aircraft fuselages and wings are often designed to include thin external skins and stiffening members attached to the skins within the interiors of the components. The integrity of such a component relies on the condition of the skin, the condition of the stiffening member, and on the condition of the bond joining the two. Ultrasonic non-destructive inspection (NDI) technologies are emerging so that in situ inspections of a vehicle and its components can be accomplished with little or no disassembly.
Ultrasonic NDI technologies are available for inspecting an external skin of a component of a vehicle by disposing a transducer and a sensor along the skin. For example, an apparatus for evaluating the stiffness of a composite plate is described in the U.S. Pat. No. 6,920,790, issued to Huang et al. on Jul. 26, 2003, the contents of which patent is incorporated herein by reference. In the identified patent, a transducer imparts energy into a plate causing a wave to propagate along the plate and the wave is detected by a sensor disposed along the plate. The velocity of the wave is determined and a material stiffness parameter for the plate is calculated based on the velocity. Such calculations are useful in inspecting the condition of a planar or smoothly contoured material such as the external skin of an aircraft fuselage.
Ultrasonic NDI technologies are available for inspecting a stiffening member by disposing a transmitter and a sensor on opposing sides of a stiffening member that is to be inspected. Such technologies are described, for example, in the co-pending U.S. patent application Ser. No. 10/752,890 of Bossi et al., the contents of which patent application, and the contents of the publication thereof, namely United States patent application publication number US2005/0145033 A1, published Jul. 7, 2005, are incorporated herein by reference. In the identified patent application publication, ultrasonic transmitters and sensors disposed within the interior of a structure are magnetically coupled to an apparatus disposed on the exterior of the structure. The transmitters and sensors move within the structure in concert with movements of the external apparatus. This arrangement appears to be useful in a situation where the movement of equipment within the interior of a structure is permitted. A vehicle component such as the fuselage or a wing of an airplane, however, typically has an interior that is partially or wholly obstructed with such things as electrical wires, ventilation ducts, actuating cables, hydraulic control lines, fuel lines, and insulation materials. Thus, inspecting stiffening members in the obstructed interior of a vehicular structure by moving equipment about in the interior is not always convenient, particularly where flight schedules may be affected.
A need exists for convenient and reliable NDI devices and methods that are capable of detecting flaws defined in a stiffening member attached to a skin. Devices and methods are needed for inspecting stiffening members through a skin so that single-sided inspection can be accomplished without disposing inspection equipment within the interior of a structure.